Microwave amplifier with bypass segment

ABSTRACT

A microwave amplifier device with a bypass segment, the device operating either in active mode or in bypass mode. The bypass segment ( 2 ) comprises at least one section ( 13 ) of coupled transmission lines ( 14,15 ) whereby the isolation is increased. The amplifier unit ( 1 ) includes two LNAs arranged in parallel in a balanced configuration, and the bypass segment ( 2 ) has a relatively high input impedance in active mode, whereby a balanced operation of the amplifier unit is maintained. Consequently, a low noise factor is obtained, as well as reduced input and output reflections.

This is a nationalization of PCT/SE01/02490 filed Nov. 9, 2001 andpublished in English.

FIELD OF THE INVENTION

The present invention relates to a device for amplifying communicationsignals in a predetermined microwave frequency band above 500 MHz. Abypass segment is automatically activated when the amplifier unitbecomes inoperative, e.g. due to failure of some RF component or DCfailure, and is effectively blocked during normal operation of theamplifier unit. More particularly, the device is of the kind defined inthe preamble of appended claim 1.

BACKGROUND OF THE INVENTION AND RELATED ART

A device of the aforementioned kind is disclosed in U.S. Pat. No.5,821,811 (Allgon AB). In the known device, the amplifier unit forms aclosed loop together with the bypass segment. In case the isolationacross the bypass segment in active mode is lower than the amplifiergain, there is a risk of instability due to feedback through the bypasssegment.

In order to secure sufficient stability, the isolation in the bypasssegment should be at least 10 dB higher than the gain of the amplifierunit in active mode. When using the previously known device, which isprovided with pin diodes as switching elements, it is necessary to usemore than two switching elements, normally four switching elements toachieve an isolation of 30 dB. However, such an arrangement also leadsto a relatively high insertion loss in the bypass mode of the device,namely in the order of 2 dB. This level is a drawback, since itadversely effects the operation of the device in the bypass mode.

A related problem of an amplifier/bypass device of the aforementionedkind is the difficulty in ensuring a proper operation of the device inactive mode. Thus, it is desired to maintain a low noise factor and toavoid input/output reflections in the amplifier unit.

Such noise will appear even if two identical low noise amplifiers (LNA)are inserted in a balanced configuration between an input hybrid couplerand an output hybrid coupler, as is previously known per se from U.S.Pat. No. 4,825,177 (General Instrument Corporation). This is due to thefact that the bypass segment will alter the balance of the amplifierunit.

SUMMARY OF THE INVENTION

Accordingly, a general object of the present invention is to provide animproved device of the aforementioned kind, which operates properly inthe active mode as well as in the bypass mode.

More specifically, a first object of the invention is to increase thebypass isolation in the active mode and to reduce the insertion loss inthe bypass segment in the bypass mode.

A second specific object of the invention is to reduce the noise factorand to reduce input/output reflections in the amplifier unit, whenoperating in active mode.

The above mentioned first object is achieved for a device having thefeatures stated in claim 1. Thus, the bypass segment of the deviceaccording to a first aspect of the invention comprises at least onesection of coupled transmission lines, whereby the isolation againstfeedback through the bypass segment is improved.

By this measure, it is sufficient to include only two switching elementsand still obtain a drastically improved isolation of about 55 dB (whenusing in diodes as in the prior art device) in the active mode and abypass insertion loss of only 1 dB in the bypass mode.

According to a second aspect of the invention, the above-mentionedsecond object is attained for a device having the features stated inclaim 6.

Preferably, the input impedance of the bypass segment, in said activemode of the device, is at least an order of magnitude higher than theinput impedance of the amplifier unit (typically 50 ohm). In this way,the two low noise amplifiers, arranged in a balanced configuration toform the amplifier unit, are balanced through a wide frequency band,even for frequencies above the predetermined microwave frequency band.Advantageously, such a relatively high input impedance of the bypasssegment (in active mode) is combined with the above-mentioned section ofmutually coupled transmission lines. Accordingly, it is hereby possibleto achieve an increased isolation and stability, a low noise factor anda reduction of the input/output reflections in the amplifier unit whenoperating in active mode, as well as a reduced insertion loss in thebypass segment when operating in bypass mode.

These and other features will be apparent from the detailed descriptionbelow.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be explained more fully below with reference to theappended drawing which illustrates a preferred embodiment of the deviceaccording to the invention.

The single drawing FIGURE illustrates schematically an amplifier devicewith a bypass segment in accordance with the present invention (on aprinted circuit board which is not shown on the drawing).

DESCRIPTION OF A PREFERRED EMBODIMENT

The circuitry shown on the drawing comprises an amplifier unit 1 with abypass segment 2 for use, e.g., in combination with an antenna, duplexmeans, and band-pass filters at the top of an antenna mast or tower in acellular mobile telephone system, as explained in the above-mentionedU.S. Pat. No. 5,821,811.

The amplifier unit 1 has the task of amplifying received communicationsignals, in a predetermined microwave frequency band in a frequencyrange above 500 MHz, before these signals become significantlyattenuated along a transmission line (not shown) to a base stationlocated adjacent to the bottom of the antenna mast or tower.

The amplifier unit 1 is inserted between an input port 3 and an outputport 4, the bypass segment 2 being arranged in parallel to the amplifierunit 1, likewise between the input and output ports 3,4.

As previously known per se (see the above-mentioned U.S. Pat. No.4,825,177), the amplifier unit comprises two low noise amplifiers (LNA)5,6 arranged in parallel between an input hybrid coupler 7 and an outputhybrid coupler 8. An input hybrid port 7 a is connected to the inputport 3 via a switching element 9, whereas an output hybrid port 8 d isconnected to the output port 4 via a switching element 10. An isolatedport 7 c of the input hybrid coupler 7 is connected to ground via a 50ohm impedance element 11, and the other output port 8 b is likewiseconnected to ground via a 50 ohm impedance element 12.

So, the two amplifiers 5,6 are single-ended and are coupled between theinput and output ports 3,4 in a fully balanced configuration. In thenormal operation of the device, during which the four switching elements9,10,16,21 are controlled to a very low impedance value, the inputsignal appearing on the input hybrid port 7 a is coupled in equalproportions to the hybrid output ports 7 b,7 d, whereas the outputsignals from the amplifiers 5,6 are coupled from the hybrid ports 8 a,8c to the hybrid output port 8 d and the output port 4 via the switchingelement 10. Preferably, the hybrid couplers 7 and 8 are constituted byLange couplers or modified Lange couplers, e.g. of the kind described inthe Swedish patent application concerning a “4-port hybrid coupler”filed by same applicant, Allgon AB, on the same day as the presentpatent application. However, it is also possible to use any other 3 dB90 degrees hybrid coupler.

Thanks to the balanced configuration of the two low noise amplifiers5,6, the amplifier unit 1 is operative in a relative broad frequencyband with a substantially constant gain.

According to the present invention, the bypass segment 2 is designed soas to increase the stability of the device and also to secure asubstantially balanced operation of the amplifier unit in spite of thebypass segment.

In order to achieve a good stability and to avoid a possible feed-backthrough the bypass segment 2, the latter is provided with two sectionsof coupled transmission lines, a first section 13 including a firsttransmission line 14 and a second transmission line 15. The firsttransmission line 14 is connected between the input port 3 of the deviceand a switching element 16, the impedance of which is automaticallyand/or remotely controllable, e.g. by a control circuit (not shown)located in the amplifier/bypass device or at an associated base station.

As disclosed in the above-mentioned U.S. Pat. No. 5,821,811, such acontrollable switching element may be constituted by a diode, a bipolartransistor, a field effect transistor or an electromechanical relay. Inany case, the switching element 16 is switched between a very highimpedance, such as 5 kohm, and a relatively low impedance, such as 1-10ohm.

When the switching elements 9,10,16,21 are controlled to a state of veryhigh impedance, which is the case when the amplifier unit is inoperablefor one reason or another (bypass mode), e.g. due to a failure of a RFcomponent or a failure of the DC current driving the unit, thetransmission line 14 will be electrically open ended (its length being ¼of the wavelength possibly added with a number N of half wavelengths,N=0,1,2, . . .) and will therefore couple strongly to the adjacent,parallel transmission line 15. The wavelength refers to a frequencywithin the microwave frequency band being used, e.g. a central frequencyin such a band.

A communication signal applied to the input port 3 will thereforepropagate via the first section 13 of coupled transmission lines 14,15,via a third, centrally located bypass segment section 17, and via asecond section 18 of coupled transmission lines 19,20. The lattersection 18 is identical to the first section 13 and the transmissionline 19 is connected between the output port 4 of the device and aremotely controllable switching element 21 similar or identical to theelement 16. The four switching elements 9,10,16,21 are controlled insynchronism so as to be either in a high impedance state or in a lowimpedance state at the same time.

However, when the amplifier unit 1 operates normally (active mode), thefour switching elements 9,10,16,21 are controlled to a state ofrelatively low impedance, whereby the bypass segment is cut off. Aninput signal on the input port 3 will then propagate through theamplifiers 5,6 to the output port 4. In the illustrated embodiment, theisolation across the bypass segment 2 is very high, approximately 55 dB,and there is no risk of feedback.

On the other hand, in the bypass mode the insertion loss in the bypasssegment 2 is very low, approximately 1 dB.

Furthermore, in active mode, thanks to the low impedance of theassociated switching elements 16 and 21 in the bypass segment, thebypass input impedance in this active mode is relatively high, wherebythe balanced operation of the two low noise amplifiers 5,6 ismaintained. Irrespective of the state of the hybrid couplers 7,8, such abalanced operation will be retained since the effective input and outputloads of the amplifiers 5,6 are unaffected by the bypass segment.Consequently, the noise factor is very low, and the input and outputreflections are likewise low.

1. A device for amplifying communication signals in a predeterminedmicrowave frequency band above 500 MHz, comprising a printed circuitboard, PCB, with transmission line segments arranged between an inputport and an output port, an amplifier unit with at least one low noiseamplifier, LNA, a bypass segment extending in parallel to said amplifierunit, and switching elements for activating said bypass segment in abypass mode of the device, in case said amplifier unit becomesinoperable, and for effectively blocking the bypass segment in an activemode of the device, said bypass segment comprising a series of at leasttwo segment sections having at least one junction point connected to anassociated one of said switching elements, characterized in that saidbypass segment comprises at least one section of coupled transmissionlines disposed adjacent to each other so as to provide a strongelectromagnetic coupling therebetween, whereby the isolation againstfeedback through the bypass segment is improved.
 2. The device definedin claim 1, wherein said bypass segment comprises two bypass segmentsections each comprising a section of coupled transmission lines, afirst transmission line in a first section being inserted between saidinput port of the device and an associated switching element, and asecond transmission line in said first section being connected toanother one of said series of bypass segment sections, a firsttransmission line in a second section being inserted between said outputport of the device and an associated switching element, and a secondtransmission line in said second section being connected to another oneof said series of bypass segment sections.
 3. The device defined inclaim 2, wherein said second transmission lines in said first and secondsegment sections are conductively connected to each other.
 4. The devicedefined in claim 3, wherein said second transmission lines are connectedto each other via a third bypass segment section.
 5. The device definedin claim 1 wherein each of said transmission lines has a length ofsubstantially a quarter wavelength.
 6. The device defined in claim 1,wherein the impedance of said switching elements is remotelycontrollable.
 7. A device for amplifying communication signals in apredetermined microwave frequency band above 500 MHz, comprising aprinted circuit board, PCB, with transmission line segments arrangedbetween an input port and an output port, an amplifier unit with atleast one low noise amplifier, LNA, a bypass segment extending inparallel to said amplifier unit, and switching elements for activatingsaid bypass segment in a bypass mode of the device, in case saidamplifier unit becomes inoperable, and for effectively blocking thebypass segment in an active mode of the device, said bypass segmentcomprising a series of at least two segment sections having at least onejunction point connected to an associated one of said switchingelements, characterized in that said amplifier unit comprises a pair ofidentical low noise amplifiers LNAs arranged in parallel in asymmetrical, balanced configuration, including an input hybrid couplerconnected to said input port and an output hybrid coupler connected tosaid output port, and said bypass segment has an input impedance in saidactive mode of the device, which is substantially higher than the inputimpedance of the device, at said input port, whereby a balancedoperation of said amplifier unit is maintained in said active mode. 8.The device defined in claim 7, provided with a bypass segment comprisingat least one section of coupled transmission lines, whereby theisolation against feedback through the bypass segment is improved. 9.The device defined in claim 7, wherein said input and output hybridcouplers are connected to said input and output ports via associatedswitching elements.
 10. The device defined in claim 7, said two LNAs aresingle-ended.
 11. The device defined in claim 7, wherein the inputimpedance of the bypass segment is at least an order of magnitude higherthan the input impedance of said amplifier unit.